Fix crash when looking up _importlib_modulespec names (#3203)

Author: pkch

mypy/fixup.py

@@ -15,6 +15,8 @@
 )
 from mypy.visitor import NodeVisitor
 
+from mypy.semanal import rev_module_rename_map
+
 
 def fixup_module_pass_one(tree: MypyFile, modules: Dict[str, MypyFile],
                           quick_and_dirty: bool) -> None:
@@ -241,6 +243,7 @@ def lookup_qualified(modules: Dict[str, MypyFile], name: str,
 
 def lookup_qualified_stnode(modules: Dict[str, MypyFile], name: str,
                             quick_and_dirty: bool) -> Optional[SymbolTableNode]:
+    name = rev_module_rename_map.get(name, name)
     head = name
     rest = []
     while True:

mypy/semanal.py

@@ -120,6 +120,8 @@
     '_importlib_modulespec.Loader': 'importlib.abc.Loader'
 }
 
+rev_module_rename_map = {v: k for (k, v) in module_rename_map.items()}
+
 # Hard coded type promotions (shared between all Python versions).
 # These add extra ad-hoc edges to the subtyping relation. For example,
 # int is considered a subtype of float, even though there is no